Modification of photochemical reactivity by incorporation of organic molecules in organised media and photocycloaddition reactions ....

Abstract

The thesis entitled (i) "Modification of Photochemical Reactivity by Incorporation of Organic Molecules in Organised Media" (ii) "Photocycloaddition Reactions of Thiocoumarin" consists of five chapters. Chapter 1 provides a comprehensive account of the structure and properties of micelles, cyclodextrin inclusion compounds and their influence on organic reactions. Environmental effects on thermal and photochemical reactions have been presented with special reference to their relative reactivity in organic solvents. The need for a systematic investigation to understand the properties of organised media is stressed. Chapter 2 deals with the effect of micelles on the photochemical behaviour of benzoin alkyl ethers (1–5, Scheme S) and ?-alkyl deoxybenzoins 6 and 7. Photolysis of 1–5 in organic solvents viz., benzene and methanol yields products derived from only the type I ?-cleavage. However, irradiation of 1–3 in micelles leads to a large change in the product distribution, giving type II hydrogen abstraction (both cyclisation and elimination) products in addition to ?-cleavage products. Remarkable difference in photobehaviour of 4 and 5 is observed. However, the influence of the micellar media on the photobehaviour of ?-alkyl deoxybenzoins 6 and 7 was small. The importance of 'cage effect' in controlling the product distribution was evident from its dependence on micellar size and on the occupancy number. The study reveals that micellar interface can be utilised to control the conformations of organic molecules. Chapter 3 summarises the photochemical reaction conducted in aqueous media. Photolysis of alkyl cinnamates (8–12) in water gave 6-truxinates in near quantitative yields. Dimerisation of alkyl cinnamates at such a low concentration (10?³ M) is attributed to hydrophobic association of these solutes. At such low concentration, photolysis in benzene and methanol yields only cis-trans isomerisation. This study demonstrates the potential of the commonly available water as a solvent for photoreactions and stresses the need for increased appreciation of its utility. Chapter 4 presents the effect of cyclodextrin encapsulation on the photo-Fries rearrangement of benzyl phenyl ethers (13–16). Photochemical studies of benzyl phenyl ethers in homogeneous solution are briefly reviewed. In organic solvents, direct excitation of ether 13 results in the formation of ortho- and para-benzylated phenols and meta-substituted phenyl benzyl ethers (14–16) produce two ortho- and one para-benzylated phenols. High ortho selectivity is observed for ether 13 in both aqueous and solid ?-cyclodextrin complexes. This selectivity was lowered in ?-cyclodextrin. Meta-substituted phenyl benzyl ethers 14 and 15 did not show much regioselectivity as expected. But 16 exhibited interesting results. It gave 6-benzyl-3-hexyloxyphenol as the major product in both aqueous and solid ?-cyclodextrin complexes. The observed ortho selectivity is explained on the basis of the intramolecular spacer effect. Chapter 5 presents the survey of the literature on the photochemistry of thiocarbonyls and the photocycloaddition reactions of thiocoumarin 17. The main features of the cycloaddition reactions and differences in reactivity from the lower and upper excited states of thiocarbonyls are highlighted. The need for a systematic investigation to understand the excited-state behaviour of thioenones is stressed. This chapter also deals with photophysical studies carried out on thiocoumarin. The important features recognised in the photophysical behaviour of thiocoumarin are: (i) phosphorescence from lowest triplet state, (ii) efficient quenching of triplet states by oxygen and ground state thiocoumarin (self-quenching), and (iii) high efficiency of intersystem crossing. An important aspect of this chapter is the cycloaddition reactions with electron-deficient and electron-rich olefins. The electron-rich and electron-deficient olefins add to thiocoumarin 17 upon both n?* and ??* excitation. Cycloaddition occurs to the thiocarbonyl chromophore. Thietane formation is non-stereospecific and site-specific. The possible routes for the interaction of thiocoumarin and olefin for the formation of thietane are discussed. A diradical and an exciplex are presumed to be involved in the addition process. Interestingly, the observed photochemical behaviour of thiocoumarin towards the olefins is quite unique and is different from that of coumarin. Appendix compares the photochemical behaviour of alkyl cinnamates (8–10) and stilbenes complexed to ?-cyclodextrin. It was anticipated that the cyclodextrin cavity might impose certain constraints on the geometric isomerisation of alkyl cinnamates. The behaviour of cinnamate esters in organic solvents and in cyclodextrin are identical, i.e., the photostationary state is the same in both the media (50% cis and 50% trans). Unlike for esters, excitation of cyclodextrin complexes of either cis- or trans-stilbene resulted in a photostationary state enriched in trans isomer. The above results are rationalised on the basis of the influence of the cyclodextrin cavity on the decay ratio of the twisted olefins.